Kit found a category called "Windows Strata" on Microsoft’s Professional Developers Conference (PDC) site (screenshot below), which would seem to indicate that this is a possible name for the OS, or at least a codename (like longhorn was for Vista). The word "strata" carries some cloud-like tendencies.

"Either ‘strata’ refers to ‘stratosphere’ – the second layer in the Earth’s atmosphere above most clouds, or ‘stratus’ – weather jargon for a type of cloud," explains Long Zheng at I Started Something. "Somehow I doubt it’s the latter since stratus is the type which casts a low gray blanket over an entire city, turning day into darkness and gloom. Probably not the association they were going for. On the other hand, stratosphere is above the rest."

Looking at the link provided by Zheng, the Windows Strata category seems to have mysteriously disappeared. When searching for "Windows Strata" on Google, the first result is a UK window company called "Strata Window World". If Microsoft is considering the Strata name for their next OS, they might want to take this into consideration, especially after the recent heat over the G.ho.st Operating System "no walls" incident.